Transition metal catalyst complexes play important roles in many areas of chemistry. Catalyst complexes are recognized to be influenced by the characteristics of the transition metal and those of the associated ligands. For example, structural features of the ligands can influence reaction rate, regioselectivity, and stereoselectivity in reactions involving the catalyst complexes. For example, in coupling reactions, electron-withdrawing ligands can be expected to slow oxidative addition to, and speed reductive elimination from, the metal center; and, conversely, electron-rich ligands can be expected to speed oxidative addition to, and slow reductive elimination from, the metal center.
Although phosphine-ligated Pd(0) complexes constitute the active catalyst in many reactions, such complexes are usually difficult to prepare and extremely air-sensitive. Tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3), which was developed to serve as a stable source of Pd(0), includes coordinating dibenzylideneacetone ligands that can significantly retard the formation of the actual active catalyst complex and/or diminish its ultimate reactivity. The use of a Pd(II) salt, such as Pd(OAc)2, which circumvents problems of precatalyst instability, requires in situ reduction in order to generate the active Pd(0) complex. In light of the complications in forming phosphine-ligated Pd(0) complexes, precatalyst scaffolds constituting the source of Pd and phosphine ligand were developed. See FIG. 1. These precatalysts formed the active, monoligated Pd complex under mild conditions and without the need for exogenous additives.
However, working with known precatalysts can be problematic. For example, the three-step preparation of precatalyst 1 (FIG. 1) involves the handling of sensitive organometallic intermediates and is not amenable to large-scale production. Additionally, precatalyst 1 is prone to decomposition in solution after a few hours and is not compatible with bulkier ligands, such as tBuBrettPhos, RockPhos, AdBrettPhos, and Me4tBuXPhos. See FIG. 10. Precatalyst 2, which can be prepared relatively simply, is not widely suitable; for example, it cannot be formed with bulkier ligands, such as BrettPhos, tBuXPhos, tBuBrettPhos, RockPhos, AdBrettPhos, and Me4tBuXPhos; additionally, it does not exhibit prolonged stability in solution.
There exists a need for a new class of air-stable, moisture-stable, solution-stable, one-component Pd precatalysts that may be activated under standard reaction conditions and ensures the formation of the active complex, L1Pd(0), with a wide range of ligands.